Paul Antony Jarvis , Grant Caldwell, Chris Noble, Yasuo Ogawa, Chris Vagasky

The 15 January 2022 eruption of Hunga Tonga-Hunga Ha’apai (HTHH) significantly impacted the Kingdom of Tonga as well as the wider Pacific region. The eruption column attained a maximum height of 58 km whilst the umbrella cloud reached a diameter approaching 600 km within about 3 hours. The intensity of volcanic lightning generated during the eruption was also unprecedented, with the Vaisala Global Lightning Database (GLD360) recording over 3 × 10^5 strikes over a two-hour period. We have combined Himawari-8 satellite imagery with the spatiotemporal distribution of lightning strikes to constrain the dynamics of umbrella spreading. Lightning was initially concentrated directly above HTHH, with an areal extent that grew with the observed eruption cloud. However, about 20 minutes after the eruption onset, radial structure appeared in the lightning spatial distribution, with strikes clustered both directly above HTHH and in an annulus of radius ~ 50 km. Comparison with satellite imagery shows that this annulus coincided with the umbrella cloud front. The lightning annulus and umbrella front grew synchronously to a radius of ~ 150 km before the umbrella cloud growth rate decreased whilst the annulus itself contracted to a smaller radius of about 50 km again. We interpret that the lightning annulus resulted from an enhanced rate of particle collisions and subsequent triboelectrification due to enhanced vorticity in the umbrella cloud head. Our results demonstrate that volcanic lightning observations can provide insights into the internal dynamics of umbrella clouds and should motivate more quantitative models of umbrella spreading.